Phytoplankton community and physical-chemical characteristics of water in the public reservoir of Cruzeta, RN, Brazil

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water in the public reservoir of Cruzeta, RN, Brazil

Chellappa, NT.

a

*, Borba, JM.

a

and Rocha, O.

b

a_{Pós-graduação em Bioecologia aquática, Departamento de Oceanografia e Limnologia,} Universidade Federal do Rio Grande do Norte – UFRN,

Av. Via Costeira, CEP 59014-100, Natal, RN, Brazil

b_{Departamento de Ecologia e Biologia Evolutiva, Universidade Federal de São Carlos – UFSCar,} Rodovia Washington Luis, Km 235, CEP 13564-905, São Carlos, SP, Brazil

*e-mail: chellappa@dol.ufrn.br

Received September 21, 2006 – Accepted March 9, 2007 – Distributed August 31, 2008 (With 8 figures)

Abstract

The Phytoplankton community and the abiotic factors of the Cruzeta reservoir were studied at three depths, sur-face, middle (2 m) and bottom, from September, 2004 to June, 2005, in order to characterize the environment and assess the possible factors that influence the compositional change of phytoplankton. Ninety species belonging to 6 classes (Chlorophyceae, Bacillariophyceae, Cyanophyceae, Dinophyceae, Chrysophyceae and Euglenophyceae) were identified with 66 and 80 taxonomic units in the dry and rainy season. The most representative class in terms

of species richness was Chlorophyceae and dominated by Scendesmus quadricauda, Oocystis sp. and Chlorella sp.

The group cyanobacteria were represented by 18 species of diverse morphological characteristics and the dominance

ofCylindrospermopsis raciborskii in September 2004. The other major group, Bacillariophyceae is represented by

21 species with the predominance of Aulacoseira granulata in mid-column and bottom waters. The other dominant

species was Phacus acuminatus of Euglenophyceae. The species diversity and evenness indices were high, moderate

and low in relation to the three hydroperiod registered during the 2004-2005 annual cycle. The reservoir exhibits high

electrical conductivity (290-550 MS.cm–1_{), alkaline pH (7.3-9.4), mean temperature of 28 °C, varying concentrations}

of dissolved oxygen (3.29-7.6 mg.L–l_{) and the greatest concentration of nutrients at the bottom (orthophosphate,}

0.22-0.62 mg.L–1_{) with the general tendency of oligo-mesotrophic status during sampling periods. The chlorophyll}

a fluctuated to a minimum of 1.34 Mg.L–l at the bottom in April, 2005 and a maximum of 14.3 Mg.L–l_{in mid-column}

water in September, 2004. The reservoir is characteristically an oligo-mesotrophic environment.

Keywords: Cruzeta RN reservoir, hydroperiod, phytoplankton, chlorophyll a, nutrients.

Comunidade fitoplanctônica e características físico-químicas

do açude público de Cruzeta, Rio Grande do Norte, BR

Resumo

A comunidade fitoplanctônica e os fatores abióticos do açude Cruzeta foram estudados em três profundidades, superfí-cie, meio (2 m) e fundo, de setembro, 2004 a junho, 2005, com o intuito de caracterizar e avaliar os possíveis fatores que influenciam as mudanças composicionais do fitoplâncton. Noventa espécies pertencentes a 06 classes (Chlorophyceae, Bacillariophyceae, Cyanophyceae, Dinophyceae, Chrysophyceae e Euglenophyceae) foram identificadas com 66 e 80 unidades taxonômicas nas estações de seca e de chuvas. A riqueza de espécies foi classe Chlorophyceae, com

dominância de Scenedesmus quadriculata,Oocystis sp. e Chlorella sp. O grupo das Cyanophyceae foi representado

por 18 espécies de características diversas com dominância de Cylindrospermopsis raciborskii em Setembro 2004.

Em seguida, foi encontrado o grupo das Bacillariophyceae, representado por 21 espécies com a predominância de

Aulacoseira granulata no fundo d’água. Outra espécie dominante foi Phacus acuminatus das Euglenophyceae. Os

ín-dices de diversidade e similaridade foram classificados como alto, moderado e baixo em relação aos três hidroperíodos

registrados, durante o ciclo anual de 2004-2005. O reservatório exibiu alta condutividade elétrica (290-550MS.cm–1_),

pH alcalino (7.3-9.4), temperatura média de 28 °C, concentrações de oxigênio dissolvido variando entre 3.29 mg.L–1_a

7.6 mg.L–1_{e maior concentração de nutrientes no fundo do reservatório (ortofosfato, 0.22-0.62 mg.L}–1_{), com}

ten-dência geral ao estado oligo-mesotrófico durante o período de amostragens. A clorofila a flutuou de um mínimo de

1.34Mg.L–1_{no fundo em Abril, 2005 e um máximo de 14.3 Mg.L}–1_{no meio da coluna d’água em Setembro, 2004. O}

reservatório é um ambiente oligo-mesotrófico característico.

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1. Introduction

The Phytoplankton biomass in reservoirs depends on various interrelated physical, chemical and biologi-cal factors (Kimmel et al., 1999) and is subject to the action of the pulses produced in the system that may be of natural origin, such as precipitation, wind and the in-flux of the river, or anthropogenic (Tundisi et al., 1999). Therefore, the phytoplanktonic communities display a potential for change that may lead to the substitution, re-moval or addition of species. Alterations in species rich-ness are mainly due to the variability of abiotic factors, such as climactic tendencies or short-term climatic vari-ations (Cody, 1996). Freshwater phytoplankton ecology studies in Brazil include reservoirs, natural lakes coastal lagoons, floodplain lakes, rivers and fish ponds. Nearly 75% of the studies come from South and Southeastern region of Brazil based on both descriptive and experi-mental approaches (Barbosa et al., 1995). The most important contributions are based on the general phy-toplankton structure, diversity to cyanobacterial domi-nance and their toxin producing capacity associated with eutrophic conditions (Azevedo et al., 1994; Bicudo et al., 1999; Huszar et al., 2000). By comparison, relatively few studies have been conducted in the Northeast (Henry, 1999). Although limited in number of publications, the investigations on the ecology of phytoplankton that have been carried out over a period of 15 years indicate the threat posed by the recurrence of two

cyanobacte-ria, Microcystis aeruginosa and Cylindrospermopsis

raciborskii (Bouvy et al., 2000 and 2003; Chellappa,

1990; Chellappa and Costa, 2003; Chellappa et al., 1996). The synoptic studies researched in Pernambuco state, indicate that the high temperature, alkaline pH, long water retention time and eutrophic to hypereu-trophic status and luxury uptake of nitrogen and phos-phorus by dominant cyanobacterial species determined phytoplankton composition in 39 Brazilian northeast reservoirs. The research also revealed the fast spreading

nature of invasive cyanobacterium, Cylindrospermopsis

raciborskii (Bouvy et al., 2000; Bouvy et al., 2003).

In Rio Grande do Norte, studies performed with limnetic phytoplankton were intensified in the 1990s. Chellappa et al. (1996) assessed semi-arid ecosystems in Rio Grande do Norte, comparing the phytoplankton composition of oligotrophic and eutrophic lakes and ob-serving the diversity of species related to the trophic sta-tus of Lakes Urubu and Extremoz. Chellapa et al. (2000) studied the phytoplankton community of São Paulo do Potengi reservoir, a salinized fresh water, where the spa-tial-temporal effect of eutrophication was verified,

high-lighting the alternating dominance between Aulocoseira

granulata (Bacillariophyceae) and Spirulina platensis

(Cyanobacteria). Studies performed at Armando Ribeiro Gonçalves reservoir in Açu, RN and at Marechal Dutra (Gargalheiras) reservoir in Acari, RN (Chellappa et al., 2000, Chellappa and Costa, 2003) also demonstrated a high degree of water eutrophication, which stimulated

the dominance of toxic cyanobacteria. Medeiros (2005) studied the phytoplankton composition of the public res-ervoir of Cruzeta, RN, in which it was detected the

pres-ence of potentially toxic cyanobacteria, such as Anabaena

circinalis and Cylindrospermopsis raciborskii.

There are two ecological hypotheses successfully tried by experimental basis to determine phytoplankton growth and biomass production: trophic cascade theory, which indicates how a strong reduction of the fish stock leads to marked increase in the zooplankton commu-nity. Many phytoplanktivorous zooplankton graze down phytoplankton biomass to a low level (Meijer et al., 1994; Nogueira et al., 2005). The second hypothesis is based on bottom-up theory where the nutrients from sediment suspension result in positive feedback to stimulate bloom formation and high phytoplankton bio-mass (Carpenter et al., 1985; Pinto-Coelho et al., 2005). Shallow water reservoirs of northeast Brazil are mostly governed by drought polygon where the low annual rainfall and high evaporation rate coupled with reser-voir draw down (flushing) determine the phytoplank-ton structure (Bouvy et al., 2003; Chellappa and Costa, 2003). The purpose of the present study was to character-ize the phytoplankton community of the public reservoir of Cruzeta in a vertical profile during a drought-rainfall cycle in 2004-2005 and to find out which factor or fac-tors determine community structure, diversity and chlo-rophyll biomass.

2. Material and Methods

The Public Reservoir of Cruzeta is located in the mu-nicipality of Cruzeta in the Western Seridó region of the state of Rio Grande do Norte (geographic coordinates 06° 24’ 42’’ S and 36° 47’ 23’’ W) (Figure 1).

It is a shallow reservoir of great importance for the municipality, since it provides water for irrigation and represents its only water source. The morphometric characteristics of the reservoir are: water holding

capac-ity 35,000,000 m3_{, discharge rate 0.174 m}3_{/s, volume of}

water during the study period 26,010,000 m3_{with a}

theo-retical renewal time of around 180 days, maximum depth 11 m and mean depth of 4.5 m.

Samples were collected monthly from September, 2004 to June, 2005 at a fixed collection station with a Van Dorn bottle (5 L) and at three depths: surface (0 m), 2 m and bottom (6 m). The following parameters were measured: pH, temperature, electrical conductivity and dissolved oxygen (WTW Multiparameter Multi 340i). Nutrient analyses such as, nitrate (Golterman et al., 1978), orthophosphate (APHA, 1985), ammonium

(Golterman et al., 1978) and chlorophyll-a

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of species (Wehr and Sheath, 2003). The quantitative measurements of phytoplankton individuals were done using the sedimentation technique and counting with the help of a Sedgwick Rafter chamber. The Pearson correla-tion test was used to assess relacorrela-tions among groups and species of phytoplankton with environmental variables using the Statistics 6.0 program.

3. Results

The recorded total annual rainfall (mean of 2004-2005 was 328 mm) is typical of the semiarid northeast region reaching a peak in March 2005 and minimum in the dry period (September to December) (Figure 2). Table 1 presents the data related to optical characteristics (euphotic zone and turbidity levels). Mean transparency during the dry and rainy periods was 0.43 and 0.57 m, respectively, but more light penetrated the water column during the dry period compared to the wet period in re-sponse to the clear and turbid nature of the shallow water in the Cruzeta reservoir (Table 1).

The mean annual water temperature was 27 °C, with a maximum surface value (30.05 °C) in February and a minimum bottom value (25.2 °C) in June, 2005. This month showed the lowest temperatures across the water column. The reduced depth of the Cruzeta reser-voir determined the isothermal pattern and total

mix-ing regime durmix-ing the study period. The concentration of dissolved oxygen in the Cruzeta reservoir oscillated between 3.17 mg.L–1_{at the 2 m depth in December and}

7.60 mg.L–1_{on the surface in June (Figure 3).}

Electrical conductivity was elevated, a fact often ob-served in reservoirs in the semi-arid regions of northeast-ern Brazil, indicating a good buffered system. The low-est electrical conductivity values were observed at the onset of the drought in September (291MS.cm–1_{), and the}

highest conductivity value was registered in the middle of the water column in April (548MS.cm–1_{). The mean}

was 330MS.cm–1_{for the dry period and 435}_M_S.cm–1_for

the rainy season. The pH had a mean value of 9.06 in the dry season and 8.70 in the rainy period, always alkaline, which is evidence of a good buffering system. The low-est value (7.39) was observed in the middle of the water

Paraíba

Paraíba Atlantic Ocean

0 10 20 km

W E

N

S

Ceará

Paraíba

Cauaçu Cauaçu

1131 1131 1121 m

1121 m

São Vicente São Vicente Florânia

Florânia RN

Atlantic ocean

Figure 1.Hydrographic basin of Piranhas-Açu river and the location of Cruzeta reservoir, RN.

Precipitation

0 50 100 150 200

Sept. Oct. Nov. Dec. Jan. Feb. Mar. Apr. May

(mm)

June

Figure 2.Monthly rainfall data of Cruzeta, RN reservoir

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25 28 31

(°C)

Sept./04 Oct./04 No

v./04

Dec./04 Jan./05 Feb

./05 Mar ./05 Apr ./05 May/05 June/05 Temperature

Sept./04 Oct./04 No

v./04

Dec./04 Jan./05 Feb

./05 Mar ./05 Apr ./05 May/05 June/05 2 4 6 8 (mg.L –1) Dissolved oxygen

Sept./04 Oct./04 No

v./04

Dec./04 Jan./05 Feb

./05 Mar ./05 Apr ./05 May/05 June/05 200 400 600 ( M S.cm –1) Conductivity

Sept./04 Oct./04 No

v./04

Dec./04 Jan./05 Feb

./05 Mar ./05 Apr ./05 May/05 June/05 6 8 10 pH

Surface Mid-column Bottom

Figure 3. Seasonal variations of Temperature, Dissolved Oxygen, Conductivity and pH of water samples of Cruzeta, RN reservoir.

Table 1. Secchi disc measurement, Euphotic zone and depth profile of Cruzeta, RN reservoir during the study period (September/04 to June/05).

Date Z_DS

(m) Z_max (m)

Z_euf (m)

Z_max / Z_euf K (m–1₎

Sept./04 0.48 3.5 1.44 2.17 18.75

Oct./04 0.44 3.2 1.32 2.10 20.45

Nov./04 0.42 3.0 1.26 2.97 21.43

Dec./04 0.40 3.5 1.20 2.75 22.50

Jan./05 0.52 4.2 1.56 1.56 17.31

Feb./05 0.68 4.0 2.04 1.96 13.24

Mar./05 0.60 6.5 1.20 1.50 15.00

Apr./05 0.52 5.0 1.25 1.56 17.31

May/05 0.53 4.5 1.39 2.20 16.98

Jun./05 0.54 4.0 1.42 1.85 16.67

Orthophosfate 0.0 0.2 0.4 0.6 0.8

Sept./04 Oct./04 No

v./04

Dec./04 Jan./05 Feb

./05 Mar ./05 Apr ./05 May/05 June/05 Oct./04 No v./04

Dec./04 Jan./05 Feb

./05 Mar ./05 Apr ./05 May/05 June/05 Sept./04 Sept./04

Oct./04 _Nov./04 Dec./04 Jan./05 Feb

./05 Mar ./05 Apr ./05 May/05 June/05 (mg.L –1) (mg.L –1) (mg.L –1) Nitrate 0.00 0.04 0.08 0.12 0.16

Surface Mid-column Bottom 0.15 Ammonium 0.05 0.10 0.20 0.25

Figure 4. Seasonal variation in inorganic nutrients concen-tration of water samples of Cruzeta, RN reservoir.

Chlorophyll a`

0 4 8 12 16

Sept.-04 Oct.-04 No

v.-04

Dec.-04 Jan.-05 Feb

.-05 Mar .-05 Apr .-05 May-05 June-05 ( M g.L –1)

Surface Mid-column Bottom

Figure 5. Seasonal variation of chlorophyll a concentrations in different depth profiles of Cruzeta, RN reservoir. column in January and the highest pH value (9.53) was

recorded on the surface in December (Figure 3). The variations in orthophosphate, nitrate and am-monium are presented in Figure 4. An absolute nutrient value of nitrate and phosphate when transformed into a N/P ratio demonstrated low nitrogen to phosphorus sta-tus and the one based on the Redfield ratio gave a very insignificant value frequently around 0 or below 10. Cyanobacterial abundance was incompatible to low N/P ratio in all three profiles.

Chlorophyll-a concentrations varied from 1.34Mg.L–1

at the bottom in April to 14.26 Mg.L–1_{at middle depth}

(2 m) in September, due to the presence and dominance

ofCylindrospermopsis raciborskii in the reservoir. The

greatest concentrations of chlorophyll-a were generally

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Table 2. Phytoplankton species and relative abundance distributed in the surface water of Cruzeta/RN Reservoir during the annual cycle of 2004-2005.

Relative abundance (%) P.O.

(%)

Sept. Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June

CHLOROPHYCEAE

Botryococcus braunii Kuetzing 0.71 2.5 1.45 - - 0.56 - 2.00 - - 50

Chlamydocapsa bacillus (Teiling) Fott - - - 0.56 1.02 4.67 0.51 1.65 50

Chlamydocapsa planctonica Fott - - - 0.56 - - - 0.55 20

Chorella sp. - 18.33 10.14 19.40 11.11 5.62 3.55 2.67 7.65 14.84 90

Closterium sp. - - 1.45 - - 4.49 1.02 2.00 1.02 - 50

Closterium tumidum Adlerstein - - - 0.51 - - - 10

Coelastrum cambricum Archer 0.36 1.67 - - 0.79 1.12 0.51 0.67 0.51 0.55 80

Coelastrum microporum Naegeli 1.25 2.50 1.45 2.99 4.76 1.12 1.02 0.67 1.53 1.10 100

Coleochaete pulvinata Smith - - - 1.02 - - 0.55 20

Cosmarium subcostatum Nordstedt 0.36 - - - 10

Crucigenia sp. - - - - 0.79 3.37 1.02 - - - 30

Cruciginela rectangularis (Naegeli) Komárek

0.54 - 11.59 4.48 15.08 2.25 3.55 0.33 2.55 1.65 90

Ctenocladus sp. - - - 0.51 - 0.51 - 20

Dictyosphaerium sp. 3.39 3.33 1.45 2.99 0.79 3.37 3.55 2.00 0.51 0.55 100

Euastrum spinulosum Delpant - 1.67 - - - 10

Eudorina elegans Ehrenberg - - - - 0.79 - - 0.67 0.51 0.55 40

Eudorina sp. 0.89 2.5 5.80 13.43 8.73 3.93 3.05 1.33 2.04 2.20 100

Hydrodictyon sp. - - - 0.51 - - - 10

Kirchneriella subsolitaria Smith G.S. West Prescot

- - - - 0.79 - - - 10

Neospongiococcum gelatinosum Ettl and Gartner

0.36 1.67 - - - 20

Oedogonium sp. - - - 0.51 3.30 20

Oocystis sp. 2.68 5.83 8.70 5.97 3.97 3.93 5.08 5.33 1.53 2.75 100

Pandorina sp. 1.07 1.67 5.80 4.48 3.97 12.36 2.54 0.67 0.51 2.20 100

Phacotus sp. - - - 6.74 1.02 2.67 1.02 1.10 50

Pleodorinasp. - - - 0.51 2.67 1.53 - 30

Scenedesmus obliqus (Turpin) Kuetzing 0.71 - - - 10

Scenedesmus quadricauda (Turpin) Brébisson

- - 4.35 23.88 32.54 32.58 47.21 28.67 25.00 18.13 80

Scenedesmus tibiscensis Uherkovich - 5.00 - - 1.59 1.12 0.51 0.67 - - 50

Sphaerocystis schroeteri Chodat 0.36 1.67 1.45 - - - - 0.67 - - 40

Staurastrum leptocladum Nordstedt 0.54 1.67 1.45 - - - 30

Staurastrum sp. - - - 0.51 - - - 10

Staurodesmus sp. - - - 0.51 - - - 10

Tetradesmus sp. 0.71 - - - 1.59 - 0.51 - 1.02 - 40

Tetraedron sp. - - - - 0.79 - - - 10

CYANOPHYCEAE Anabaena circinalis

Robenhorst ex Bornet et Flahault

0.36 1.67 - 1.49 0.79 2.81 0.51 1.33 1.02 0.55 90

Anabaena planktonica Brunnthaler 0.36 - - - 10

Anabaena spiroides Klebahn 0.54 1.67 - - - 20

Aphanocapsa sp. - - - 0.51 2.67 0.51 - 30

Cylindrospermopsis raciborskii (Wolosz.) Seen. and Subba

68.21 - - - 10

Dinophyceae, Chrysophyceae and Euglenophyceae), with a total of 66 in the dry period and 80 in the rainy season (Tables 2, 3 and 4). The most representative class in terms of species richness was Chlorophyceae

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Relative abundance (%) P.O. (%)

Sept. Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June

Eucapsis sp. - - - - 0.79 1.69 - 0.67 0.51 0.55 50

Gloeothece sp. - - - 0.51 - 10

Lyngbya sp. 1.43 - - - 1.02 0.67 0.51 0.55 50

Oscillatoria limosa Ag. 8.93 1.67 2.9 - - - 0.51 0.55 50

Planktolyngbya contorta (Smith) Echdrateva

0.36 1.67 - - - - 0.51 - 0.51 - 40

Planktothrix sp. - - 1.45 - - - 0.51 0.67 2.55 - 40

Pseudoanabaena catenata 0.54 - - - 1.52 3.33 4.08 1.65 50

Raphidiopsis curvata Fritsch and Rich 1.25 - - - 10

Rivularia sp. - - - 0.51 0.67 - - 20

Spirulina major Kutz 0.54 - 1.45 2.99 - - - 0.67 - - 40

Synechocystis sp. 0.36 - - - 0.51 4.00 7.65 1.65 50

Trichodesmium sp. 0.36 - - - 0.67 - 0.55 30

Woronichinia sp. - - 2.9 4.48 2.38 1.12 - - - 0.55 50

BACILLARIOPHYCEAE

Amphora ovalis Kutz - - - 0.51 2.00 2.04 0.55 40

Aulacosira granulata 1.25 13.33 20.29 2.99 0.79 1.69 2.03 2.00 3.57 3.30 100

Cocconeis sp. - - - 1.10 10

Cyclotella meneghiniana Kutz - 1.67 1.45 - 0.79 0.56 - 1.33 2.55 1.10 70

Cymbella minuta Hilse - 2.50 - - - 10

Fragilaria lapponica Grunow - - - 0.51 0.67 - - 20

Gomphoneis herculana (Ehr.) Cleve - 2.50 1.45 - - - 20

Gomphonema Augir Ehr. - - - 0.67 - 0.55 20

Gomphonema sp. - 2.50 1.45 - - - 0.51 1.33 0.51 - 50

Gyrosigma sp. - 2.50 - - - 0.51 - 20

Navicula cuspidata Kuetzing - - - 0.67 - 1.65 20

Navicula exigua (Greg.) Muller - - - 0.51 - - - 10

Navicula forcipata Grev. - - 2.90 7.46 0.79 0.56 3.55 4.67 3.06 1.65 80

Navicula lapidosa Krasske - - - 2.00 - - 10

Navicula placentula (Erh.) Kuetzing - - - 0.51 1.10 20

Navicula radiosa Kutzing - - - - 0.79 0.56 - 0.67 2.04 0.55 50

Navicula sp. 0.54 4.17 - - - - 0.51 - - - 30

Nitszchia sp. - - 1.45 - - - 2.03 0.67 2.55 - 40

Pinnularia similis Hustedt - - - 0.51 0.67 - 1.10 30

Surirella sp. - 2.5 1.45 - 1.59 - 0.51 0.67 0.51 0.55 70

Synedra sp. - - - 1.49 - - - 0.67 - - 20

DYNOPHYCEAE

Glenodinium sp. 0.36 2.5 2.9 - 0.79 - - - 1.02 - 50

Peridinium sp. 0.36 1.67 - - - - 1.02 - - - 30

CHRYSOPHYCEAE

Mallomonas producta Iwanoff 0.36 3.33 2.9 -1.49 2.38 2.25 0.51 0.67 2.55 - 90

Mallomonas striata Asmund - - - 1.69 0.51 0.67 0.51 3.30 50

Pedinella sp. - 2.5 - - - 10

EUGLENOPHYCEAE

Euglena sp. - - - 1.02 - 10

Phacus acuminatus Stokes - 1.67 - - - 4.49 2.03 3.33 10.2 26.37 60

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Table 3. Phytoplankton species and relative abundance distributed in the mid-column water of Cruzeta, RN Reservoir during the annual cycle of 2004-2005.

(%)

Sept. Oct. Nov. Dec. Jan. Fev. Mar. Apr. May June

CHLOROPHYCEAE Asterococcus superbus (Cienkowski) Scherffel

- - - 0.83 - - - 10

Botryococcus braunii Kuetzing 0.58 2.42 - - 0.46 0.47 0.41 - - 0.46 60

Chlamydocapsa bacillus (Teiling) Fott - - - 0.47 2.07 0.83 0.29 - 40

Chlamydocapsa planctonica Fott - - - 0.94 - - - 0.46 20

Chorella sp. - 25.81 16.9 9.68 18.43 9.91 3.32 3.33 4.66 12.04 90

Closterium lunula Müll - - - - 0.46 - - - 10

Closterium sp. - - - 3.30 - - 0.58 0.93 30

Coelastrum cambricum Archer 0.58 1.61 - - 1.38 - 0.41 0.42 0.29 0.93 70

Coelastrum microporum Naegeli 1.17 1.61 1.41 1.61 2.76 - 0.41 0.42 0.29 0.93 90

Coleochaete pulvinata Smith - - 1.61 1.38 0.94 - - 0.29 0.46 50

Crucigenia sp. - - - - 1.84 1.42 3.73 - 0.58 0.93 50

- 7.26 5.63 4.84 7.83 1.42 4.15 2.50 0.87 1.85 90

Ctenocladus sp. - - - 0.41 0.42 - - 20

Dictyosphaerium sp. - 2.42 1.41 1.61 0.92 1.42 0.83 0.42 - 0.93 80

Eudorina elegans Ehrenberg - - - 1.61 - - 0.41 - - 0.46 30

Eudorina sp. 2.92 3.23 5.63 8.06 1.84 5.66 7.47 1.67 1.17 0.46 100

Neospongiococcum gelatinosum Ettl and Gartner

0.58 1.61 - - - 20

Oedogonium sp. - - - 5.56 10

Oocystis sp. 2.92 2.42 8.45 6.45 0.92 4.25 2.90 4.17 1.46 4.63 100

Pandorina sp. 1.17 1.61 - 3.23 5.07 3.77 5.39 0.42 2.33 1.39 90

Phacotus sp. - - - - 4.61 4.72 1.24 0.83 0.87 1.85 60

Pleodorina sp. - - - 2.08 0.58 0.93 30

Scenedesmus ecornis Chodat - - - 0.47 - - - - 10

Scenedesmus obliqus (Turpin) Kuetzing 1.17 - - - 10

- - 7.04 35.48 34.1 30.66 43.98 20.83 16.03 17.59 80

Scenedesmus tibiscensis Uherkovich - 1.61 - - 0.94 1.24 0.42 0.58 0.93 60

Schroederia setigera (Schroder) Lemm - - - 0.29 - 10

Sphaerocystis schroeteri Chodat - 1.61 - - - 10

Staurastrum leptocladum Nordstedt 1.17 3.23 2.82 - - - 30

Staurastrum sp. - - - 1.61 - - - 10

Staurodesmus triangularis (Lagerheim) Teiling

- - - 0.29 0.46 20

Tetradesmus sp. - - 5.63 - 0.46 - - 0.83 - - 30

Tetraedron sp. - - - 0.29 - 10

0.58 1.61 1.41 1.61 0.92 0.94 0.83 - - 0.46 80

Anabaena spiroides Klebahn 0.58 1.61 - - - 20

Aphanocapsa sp. - - 1.41 - 0.92 0.94 1.24 0.42 0.29 - 60

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Sept. Oct. Nov. Dec. Jan. Fev. Mar. Apr. May June

Eucapsis sp. - - - 1.42 0.41 0.42 - 0.46 40

Gloeothece sp. 1.17 - - - 0.83 - - 20

Lyngbya sp. 4.09 - - - - 1.89 0.83 0.83 0.58 - 50

Oscillatoria limosa Ag. 1.17 1.61 1.41 - - - 0.29 - 40

Planktolyngbya contorta (Smith)

Echdrateva

- 1.61 - - - 10

Planktothrix sp. - - 1.41 - 0.46 1.42 - - - - 30

Pseudoanabaena catenata - - - 1.25 - 10

Raphidiopsis curvata Fritsch and Rich 4.09 - - - 10

Rivularia sp. - - - 0.46 10

Synechocystis sp. - - - 7.05 48.75 4.37 3.7 40

Trichodesmium sp. - - - 0.29 - 10

Woronichinia sp. - - 2.82 4.84 0.92 1.42 1.24 1.25 0.87 0.93 80

BACILLARIOPHYCEAE

Amphora sp. - - - 0.42 - 0.46 20

Amphora ovalis Kutz - - - 3.23 - - - 10

Cocconeis sp. - - - 0.29 - 10

Cyclotella meneghiniana Kutz - 1.61 1.41 - - - - 0.42 0.58 0.46 50

Diatoma sp. - - - 0.47 0.83 - 0.29 - 30

Gomphoneis herculana (Ehr.) Cleve - 3.23 - - - 10

Gomphonema sp. - 3.23 - - - 0.47 - - 0.29 - 30

Gyrosigma sp. 1.17 - - 3.23 - - - 0.42 - - 30

Navicula cuspidata Kuetzing - - - 0.29 - 10

Navicula forcipata Grev. - - - 1.61 - 0.47 - - 0.58 - 30

Navicula lapidosa Krasske - - - 2.07 - - - 10

Navicula placentula (Erh.) Kuetzing - - - 0.29 - 10

Navicula radiosa Kutzing - - - 0.58 - 10

Navicula sp. 1.17 2.42 - - 0.92 - - 0.83 1.17 0.93 60

Nitszchia sp. - - - 0.94 0.83 - 0.29 - 30

Pinnularia similis Hustedt - - - 0.29 - 10

Pinnularia subcapitata Gregory - - - 0.83 - - 10

Raphoneis sp. - - - 0.46 10

Surirella sp. - 1.61 - - - 0.42 - - 20

Synedra sp. - - 1.41 - 0.92 - - - 20

DINOPHYCEAE

Glenodinium sp. 1.17 1.61 4.23 - 0.92 - - - 40

Peridinium sp. 0.58 1.61 1.41 - - - 30

CHRYSOPHYCEAE

Mallomonas producta Iwanoff 1.17 - - 4.84 2.76 0.94 0.41 0.42 1.46 - 70

Mallomonas striata Asmund - - - - 0.92 1.42 0.41 0.42 2.04 2.31 60

EUGLENOPHYCEAE

Euglena sp. - - 5.63 - - 0.94 - - - 0.93 30

Phacus acuminatus Stokes - - - - 4.61 10.85 4.15 2.92 50.44 30.09 60

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Table 4. Phytoplankton species and relative abundance distributed in the bottom water of Cruzeta, RN Reservoir during the annual cycle of 2004-2005.

(%)

CHLOROPHYCEAE

Botryococcus braunii Kuetzing - 3.23 1.67 - 0.79 1.72 0.58 - - - 50

Chlamydocapsa bacillus (Teiling) Fott - - - 2.92 - - 0.72 20

Chorella sp. - 1.61 18.33 8.89 3.17 4.31 4.09 5.45 3.57 3.62 90

Closterium sp. - - - - 2.38 1.72 - - - - 20

Coelastrum cambricum Archer - 1.61 - - 1.59 1.72 0.58 0.61 - - 50

Coelastrum microporum Naegeli 1.54 1.61 1.67 1.11 3.17 2.59 0.58 1.21 0.89 0.72 100

Coleochaete pulvinata Smith - - - 3.97 0.86 - - 0.89 0.72 40

Crucigenia sp. - - - 4.31 1.17 0.61 - - 30

- - 3.33 2.22 3.97 1.72 1.17 2.42 - - 60

Ctenocladus sp. - - - 0.58 - - - 10

Desmidium bailey (Ralfs) De Bary 1.54 - - - 10

Dictyosphaerium sp. - 3.23 1.67 - 3.97 0.86 0.58 0.61 0.89 - 70

Eudorina elegans Ehrenberg - - - 0.58 - - - 10

Eudorina sp. 1.54 12.90 8.33 7.78 4.76 4.31 3.51 2.42 2.68 1.45 100

Oedogonium sp. - - - 0.89 55.07 20

Oocystis sp. 1.54 9.68 5.00 12.22 2.38 0.86 1.75 4.85 0.89 1.45 100

Pandorina sp. - 6.45 1.67 5.56 5.56 2.59 4.09 3.03 2.68 1.45 90

Pediastrum sp. 3.08 - 1.67 - - - 1.45 30

Phacotus sp. - - - - 1.59 1.72 - 1.82 2.68 - 40

Pleodorina sp. - 3.23 - - - 4.24 1.79 - 30

10.77 - 6.67 37.78 41.27 43.10 54.39 43.03 35.71 8.7 90

Scenedesmus tibiscensis Uherkovich - 3.23 - 2.22 - 1.72 0.58 0.61 - - 50

Schroederia setigera (Schroder) Lemm - - - 0.89 - 10

Sphaerocystis schroeteri Chodat - 1.61 - - - - 0.58 - 0.89 - 30

Staurastrum leptocladum Nordstedt 1.54 - 1.67 - - 0.86 - - - - 30

Staurastrum striolatum (Naegeli) Archer

- 1.61 - - - 10

Staurodesmus triangularis (Lagerheim) Teiling

- - - 0.89 - 10

Tetradesmus sp. - - - 2.22 - - - 10

- 1.61 - 1.11 0.79 - - - 2.68 0.72 50

Aphanocapsa sp. - - - - 0.79 - 1.75 - 1.79 - 30

4.62 - - - 10

Eucapsis sp. - - - - 2.38 - - - 0.89 - 20

Lyngbya sp. 6.15 - - - - 1.72 0.58 0.81 1.79 2.17 60

Oscillatoria limosa Ag. 1.54 1.61 1.67 - - - 30

Planktolyngbya contorta

(Smith) Echdrateva

- 1.61 1.67 - - - 20

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Pseudoanabaena catenata - - - 0.86 0.58 1.21 - - 30

Rivularia sp. - - - 0.58 - - - 10

Synechocystis sp. - - - 2.59 3.51 4.24 - - 30

Trichodesmium sp. 1.54 - - - 10

Woronichinia sp. - - 3.33 3.33 1.59 - - - 1.79 1.45 50

BACILLARIOPHYCEAE

Amphora sp. - - - 0.61 - - 10

Amphora ovalis Kutz 3.08 - - 1.11 - - - 10

Ceratoneis sp. - 1.61 - - - 10

Cyclotella meneghiniana Kutz 1.54 3.23 5.00 1.11 - 0.86 - - 1.79 - 60

Epithemia sp. 1.54 - - - 10

Fragilaria lapponica Grunow 1.54 - - - - 1.72 0.58 0.61 0.89 - 50

Gomphoneis herculana (Ehr.) Cleve - - 1.67 - - - 10

GomphonemaAugir Ehr. - - - 0.89 - 10

BACILLARIOPHYCEAE

Gomphonema sp. - - - - 0.79 - - 1.21 - 1.45 30

Gyrosigma sp. 1.54 3.23 - - - 1.72 - - - 1.45 40

Navicula cuspidata Kuetzing - - - 0.58 - 0.89 - 20

Navicula forcipata Grev. - - 1.67 3.33 - - - 20

Navicula lapidosa Krasske - - - 1.82 1.79 - 20

Navicula radiosa Kutzing - - - 0.61 2.68 0.72 30

Navicula sp. 1.54 - - - 4.09 4.24 3.57 0.72 50

Nitszchia sp. - - - 2.34 3.03 2.68 2.17 40

Pinnularia similis Hustedt - - - 0.58 0.61 - - 20

Pinnularia subcapitata Gregory 1.54 - - 1.11 - - - 20

Raphoneis sp. 1.54 - - - 10

Stauroneis anceps Ehr. 1.54 - - - 10

Surirella sp. 1.54 - 1.67 - - - 1.45 30

Synedra sp. - - - - 1.59 1.72 - - 3.57 1.45 40

DYNOPHYCEAE

Glenodinium sp. 1.54 1.61 5.00 - 1.59 - - - 40

Peridinium sp. - 1.61 - - - 10

CHRYSOPHYCEAE

Mallomonas producta Iwanoff 3.08 1.61 1.67 7.78 1.59 - 0.58 0.61 1.79 - 80

Mallomonas striata Asmund - - - 0.58 0.61 - 1.45 30

Pedinella sp. - - 3.33 - - - 10

EUGLENOPHYCEAE

Phacus acuminatus Stokes - - - - 2.38 6.9 1.75 1.82 9.82 2.17 60

P.O. (%) – Percentage of occurrence. Table 4. Continued...

80 taxons were identified at the surface, 77 at mid-depth (2 m) and 72 at the bottom. Regarding the algal class, Chlorophyceae exhibited the highest number of species at the surface and at mid-depth (34 species) during the rainy season (29 and 28 species, respectively). There was a greater number of Bacillariophyceae at the bottom of

the reservoir (23) than at the surface and mid-depth (21). These relative abundances were expressed as class wise in three vertical profiles indicating explicitly the succes-sion sequence (Figures 6, 7 and 8).

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Chlorophyta Cyanophyta Bacillariophyta Dinophyta Euglenophyta Chrysophyta Surface

September November

January

March May

June

April

February December

October 14%

83% 2%

1% 0%0%

49%

7% 32%

4%2% 6%

55% 30%

3%0% 3%

9%

78% 9%

12% 1% 0% 0%

88% 4%5%

1% 2% 0%

83% 4%

4%

6% 3%0%

79% 6%

11%

1% 1% 2%

62% 15%

19%

3% 0% 1%

49% 3%

11% 1%

18%

53% 3%

5% 13% 0%

26%

Figure 6. Phytoplankton distribution in surface waters of Cruzeta, RN reservoir from September 2004 to June 2005.

and the frequency of occurrence of phytoplankton in Cruzeta reservoir in the three vertical profiles during the annual cycle of 2004-2005. The largest amount of taxa was found in the rainy season at the surface (68) and middle (66) of the water column. Chlorophyceae domi-nated the surface during the entire study period except in September, when the Cyanophyceae dominated with

79%, the most frequent species being the potentially

tox-ic cyanobacteria Cylindrospermopsis raciborskii with a

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Chlorophyta Cyanophyta Bacillariophyta Dinophyta Euglenophyta Chrysophyta Mid-Column

September

5%2%0%1%

79% 13%

November

January

March May

October

December _February

April

June

55% 2%

31%

6% 6% 0%

1%

40%

52% 4%0%3%

71% 2%

12% 0% 7%

8% 0% 5%

76% 13%

6%

57% 34%

6% 3% 0%

50% 4%

31%

9% 6% 0% 12%

79% 4%0%4%1%

3%3% 1% 5% 4%

84%

0%

55% 6%

6%

25%

8%

was followed by Cyanophyceae and in June, when they were followed by Euglenophyceae dominance (26%). The most abundant Chlorophyceae species at the surface

wereScenedesmus quadricauda (Turpin) Brébisson and

Chorella sp.. The dominance of phytoplankton groups

in the middle of the water column in September, October, November, December and June behaved in the same manner that was observed at the surface. In January and February a dominance of Chlorophyceae

was observed, followed by Euglenophyceae, while in March Chlorophyceae (79%) was dominant, followed by Cyanophyceae (12%). Cyanophyceae dominated in April (dry period) with 52%, followed by Chlorophyceae with 31%. A growth in Euglenophyceae (50%) occurred in May, when it became dominant, particularly the

spe-cies Phacus acuminatus Stokes (relative abundance

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Chlorophyta Cyanophyta Bacillariophyta Dinophyta Euglenophyta Chrysophyta

September November

January

March May

2% 0%

3% 23%

14% 58%

52%

8%

78%

77% 56%

June

April

February December

October

76% 4%

17%

71% 1%

2% 0%

0%

0% 0%7%

75% 80%

50% 40%

3%0%2%

5%

0% 8%

8%

4%

6% 12%

20%

6%

2% 1%

2% 10%

9% 23% 9%

11%0%

0% 2%1%

2% 2% 2% 10% 6%

30%

5%0%5% Bottom

in September when there was a strong dominance of Bacillariophyceae. The percentage dominance for these phytoplanktonic species is presented in Table 5.

Table 6 presents some significant correlation between environmental variables and dominant phytoplankton groups on the spatial-temporal scale. A significant corre-lation was established for cyanobacterial abundance with water temperature, dissolved oxygen concentration and transparency during the dry period. Bacillariophyceae correlated positively to water transparency and

ortho-phosphate levels specifically at the bottom during the dry period and with pH, electrical conductivity and or-thophosphate during the wet period. The other dominant group, Chlorophyceae, positively correlated to pH, ni-trate and electrical conductivity.

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Table 5. Density of species (ind.mL–1_{) dominance at surface, mid-column and bottom waters of Cruzeta, RN reservoir during} September/04 to June/05.

Surface P.O.

(%)

Sept. Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June

CHLOROPHYCEAE

Chlorella sp. - 1.100 700 1.600 1.400 1.000 700 400 1.500 2.700 90

Coelastrum cambricum Archer 100 100 - - 100 200 100 100 100 100 80

Coelastrum microporum Naegeli 350 150 100 200 600 200 200 100 300 200 100

150 - 800 300 1.900 400 700 500 500 300 90

Dictyosphaerium sp. 950 200 100 200 100 600 700 300 100 100 100

Eudorina sp. 250 150 400 900 1.100 700 600 200 400 400 100

Oocystis sp. 750 350 600 400 500 700 1.00 800 300 500 100

Pandorina sp. 300 100 400 300 500 2.200 500 100 100 400 100

- - 300 1.300 4.100 5.800 9.300 4.300 4.900 3.300 80

CYANOPHYCEAE

Anabaena circinalis Robenhorst ex

Bornet et Flahault

100 100 - 100 100 500 100 200 200 100 90

BACILLARIOPHYCEAE

Aulacoseira granulata 350 800 1.400 200 100 300 400 300 700 600 100

Navicula forcipata Grev. - - 200 500 100 100 700 700 600 300 80

CHRYSOPHYCEAE

Mallomonas producta Iwanoff 100 200 200 100 300 400 100 100 500 - 90

Mid-Collum

CHLOROPHYCEAE

Chorella sp. - 1.600 1.200 600 4.000 2.100 800 800 1.600 2.600 90

Coelastrum microporum Naegeli 200 100 100 100 600 - 100 100 100 200 90

- 450 400 300 1.700 300 1.000 600 300 400 90

Dictyosphaerium sp. - 150 100 100 200 300 200 100 - 200 80

Eudorina sp. 500 200 400 500 400 1.200 1.800 400 400 100 100

Oocystis sp. 500 150 600 400 200 900 700 1.000 500 1.000 100

Pandorina sp. 200 100 - 200 1.100 800 1.300 100 800 300 90

- - 500 2.200 7.400 6.500 10.600 5.000 5.500 3.800 80

CYANOPHYCEAE

Anabaena circinalis Robenhorst ex

Bornet et Flahault

100 100 100 100 200 200 200 - - 100 80

Woronichinia sp. - - 200 300 200 300 300 300 300 200 80

BACILLARIOPHYCEAE

Aulacoseira granulata 400 1.350 1.600 300 300 900 100 200 200 900 100

Bottom

CHLOROPHYCEAE

Chorella sp. - 100 1.100 800 400 500 700 900 400 500 90

Coelastrum microporum Naegeli 100 100 100 100 400 300 100 200 100 100 100

Eudorina sp. 100 800 500 700 600 500 600 400 300 200 100

Oocystis sp. 100 600 300 1.100 300 100 300 800 100 200 100

Pandorina sp. - 400 100 500 700 300 700 500 300 200 90

Scenedesmus quadricauda (Turpin)

Brébisson

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Bottom P.O. (%)

Sept. Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June

BACILLARIOPHYCEAE

Aulacosira granulata 2.700 2.000 1.200 100 1.000 700 400 1.200 500 1.000 100

CHRYSOPHYCEAE

Mallomonas producta Iwanoff 200 100 100 700 200 - 100 100 200 - 80

Criteria of dominance based on Matucci and Colma (1982): 76 -100% Dominants

P.O. (%) – Percentage of occurrence. Table 5. Continued...

Table 6. Pearson correlation coefficient values between abiotic factors and phytoplankton classes (P < 0.01).

Variables Dry Season Rainy Season

Surface Mid-Column Bottom Surface Mid-Column Bottom

Temperature and Bacillariophyceae –1.00 –0.82 - - -

-Cyanophyceae and Nitrate –0.96 - - - -

-Euglenophyceae and Orthophosphate 0.98 - - - -

-Chlorophyceae and Dissolved Oxygen 0.80 –0.90 - - -

-Euglenophyceae and pH - - - 0.98 -

-Orthophosphate and

Bacillariophyiceae / Euglenophyceae

- - - 0.87 / 0.85 -

-Chlorophyceae and Transparency - 0.99 –0.97 - -

-Bacillariophyceae and Rainfall - –0.98 - - -

-Cyanophyceae and

Transparency / Conductivity

- 0.86 / –0.82 - - -

-Cyanophyceae and

Conductivity / Orthophosphate

- - - - 0.82 / 0.83

-Chlorophyceae and

Conductivity / Orthophosphate

- - - - 0.83 / –0.87

-Chlorophyceae and pH/ Conductivity - - 0.93 / 0.97 - -

-Cyanophyceae and Ammonium - - 0.97 - -

-Bacillarioficeae and Transparency - - 0.97 - -

-Bacillariophyceae and pH - - –0.99 - - 0.83

Bacillariophiceae and Orthophosphate - - 0.92 - - 0.93

Bacillariophyceae and Conductivity - - –0.99 - - 0.98

Bacillariophyceae and Nitrate - - –0.8 - -

-Table 7. Species richness, Species diversity, Dominance, Equitability and Total Density of species during the annual cycle of 2004-2005.

Months Richness Diversity Dominance Equitability Total density (ind.mL–1₎

S M B S M B S M B S M B S M B

Sept./04 6.75 5.20 6.29 2.22 2.29 3.43 0.68 0.67 0.42 1.49 1.68 2.45 28.000 17.100 6.500

Oct./04 7.68 6.33 5.54 4.44 3.75 3.99 0.18 0.26 0.32 3.01 2.68 2.97 6.000 6.200 6.200

Nov./04 6.51 4.93 5.82 4.02 3.68 3.60 0.20 0.23 0.20 2.84 2.83 2.64 6.900 7.100 6.000

Dec./04 3.66 4.48 4.05 3.35 3.39 3.13 0.24 0.35 0.38 2.85 2.70 2.54 6.700 6.200 9.000

Jan./05 6.10 6.00 5.37 3.45 3.46 3.45 0.33 0.34 0.41 2.44 2.42 2.53 12.300 21.700 12.600

Feb./05 5.88 7.17 6.15 3.53 3.57 3.46 0.33 0.31 0.43 2.49 2.37 2.45 17.800 21.200 11.600

Mar./05 10.01 6.62 7.09 3.58 3.08 3.06 0.47 0.44 0.54 2.18 2.09 2.05 19.700 24.100 17.100

Apr./05 10.30 6.85 6.40 4.50 2.75 3.41 0.29 0.49 0.43 2.74 1.84 2.36 15.000 24.000 16.500

May/05 9.55 8.82 7.41 4.30 3.13 3.89 0.25 0.50 0.36 2.65 1.94 2.61 19.600 34.300 11.200

June/05 3.35 3.63 2.75 3.35 3.63 2.75 0.26 0.52 0.55 2.15 2.39 0.66 18.200 21.600 13.800

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values of specific diversity, and equitability (similarity), richness and dominance were observed at the surface and bottom waters than at mid-column. In relation to the dry and wet seasons, there was greater diversity and less evenness during the rainy period. In September 2004, an

invasive species of cyanobacterium, Cylindrospermopsis

raciborskii, dominated both at the surface and the

mid-dle of the water column and never repeated. An elevated

dominance of Scenedesmus quadricauda, Oocystis sp.

and Oedogonium sp., members of Chlorophyceae was

also observed at the bottom in March and June of 2005. The dominance of these three species determined a re-duction in the diversity values in the dry/wet phases of the annual cycle. Phytoplankton species diversity is ad-dressed in relation to three hydroperiods with six repli-cates for each period which reflect the level of organi-zation of the phytoplankton community of Cruzeta, RN reservoir. Phytoplankton species diversity and evenness reflect important processes such as growth, sedimenta-tion, possible grazing losses and nutrient assimilasedimenta-tion, which varied markedly in relation to spatial heterogene-ity and the dry/wet annual cycle. The present study re-veals three hydroperiods, such as the dry period (Sept. to Dec. 2004), the wet period (Jan. to Feb. 2005 and Apr. to June 2005) and the peak rainy period of March 2005. The species diversity and equitability indices were great-ly reduced in March 2005 largegreat-ly due to low attenuation of light due to the accumulation of high suspended mate-rials near the surface.

4. Discussion

Shallow water reservoirs (mean depth <6 m) are dominant freshwater systems in the Rio Grande do Norte State and are more vulnerable to human impact on phy-toplankton dynamics and inland fisheries of the State (Chellappa and Chellappa, 2004). The spatio-temporal distributions of major phytoplankton taxa were quanti-fied to estimate the relative contribution to abundance, cyanobacterial bloom dynamics and chlorophyll biomass in the oligo-mesotrophic Cruzeta, RN reservoir during the annual cycle of 2004-2005. The results are discussed within the framework of shallow water freshwater eco-systems principle and to highlight how phytoplankton community structure, diversity and biomass respond to clear and turbid water conditions during the dry and wet cycles of semiarid climate (Bouvy et al., 2003).

Depth influences sinking loss of phytoplankton and its suspension to surface layer in deep water reservoirs and lakes, while the pronounced effect experienced by phytoplankton under light regime in shallow waters is because of the light attenuation coefficient of the mixed layer (Shaffer, 2004). The present study indicates that the mean depth was never more than 4 m throughout the study period, of mostly an isothermal characteristic with no marked stratification. The light regime experi-enced by the phytoplankton varied on a seasonal basis, with more light penetration in the dry than wet period

and the chlorophyll biomass linearly linked to the light regime than to phytoplankton abundance. Such findings are similar to the results of the Parelhas, RN reservoir (Chellappa et al., 2006). The polymictic nature of semi-arid reservoirs where elevated daytime and low nighttime temperatures characterize the diurnal cycle, favor impor-tant convective movements in the mixing of the water column and the consequent rise of nutrients to the water surface (Bouvy et al., 1999; Chellappa and Costa, 2003). The earlier study of Barbosa and Tundisi (1980) regis-tered such phenomena in other shallow water lakes in Brazil and they considered it as an efficient mechanism of the recycling process in tropical waters and conse-quently improved primary production. The results of nu-trient values found in the present study is akin to the re-search performed by the earlier studies of Chellappa and Chellappa (2004) for the Rio Grande do Norte State, in which the data on surface and bottom concentrations of phosphorous were generally similar, with the exception of September, when phosphorous concentration at the bottom was much greater than that at the surface.

Cruzeta reservoir of Rio Grande do Norte main-tained a high phytoplankton species richness (90 taxons) during the 2004-2005 annual cycle, which is very nearly similar to the report of Train et al. (2005) on algal species in three reservoirs in the Parana basin-Irai, Salto do Vau and Rosana, where phytoplankton was distributed in the following classes: Chlorophyceae (32), Cyanophyceae (17), Bacillariophyceae (15), Chrysophyceae (5), Zygnematophyceae (5), Xanthophyceae (3) and Dinophyceae (2). However, Huszar and Giani (2004) pointed out that the phytoplankton community might present a different pattern in vertical distribution, as a result of the physical structuring of the water mass and the capacity of algae to regulate their position through specific adaptive strategies. The present study demon-strates that Bacillariophyceae was more abundant in mid-column and bottom waters, while Chlorophyceae and Cyanobacteria are more abundant in the surface and mid-column of the reservoir mostly as a response to surface/volume ratio of these groups and the differential buoyancy mechanism.

Bicudo et al. (1999) attributed the high diversity of species found to the mixing of the water column, which changes the progress of succession to a condition of equi-librium. Thus, the presence of an unbalanced community triggers high diversity indices. Moran et al. (1997) ob-served that the biodiversity indicator is fundamental in assessing ecological stability in environments. The high diversity values obtained in the present study can be considered an indicator of the stability of Cruzeta, RN reservoirs because of two reasons: it maintained multi-species phytoplankton in the annual cycle, as evidenced by moderate indices of equitability and the restriction of high dominance index by cyanobacteria only on a rare occasion (September, 2004).

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ecosystems. Through the contribution of nutrients and materials in suspension (runoff), light penetration is re-duced and chemical conditions and mixing of the water, triggers the succession process. Giani and Figueiredo (1999), in a study performed at the Pampulha reservoir, Minas Gerias, Brazil, reported that rainfall acts as a di-luting factor and, at the same time, a disturber of aquatic communities, having a strong influence on the compo-sition of phytoplankton species as well as on total bio-mass.

Nutrient dynamics is usually linked to seasonal vari-ation and the degree of allochthonous input to the res-ervoirs. Nutrient availability in shallow lakes differed profoundly between shallow polymictic lakes to deep stratified lakes (Scheffer, 2004). The nutrient loss is con-tinuous through sinking in deep waters from epilimnion to hypolimnion, while shallow waters frequently replen-ish nutrients through efficient recycling and elevated mixing regime (Barbosa and Tundisi, 1980; Scheffer, 2004). The low N/P ratio coupled with high temperature favor cyanobacterial blooms (Smith, 1983), but in the present study, such observation was made only during

a single incidence of Cylindrospermopsis raciborskii

bloom in September 2004 while the rest of the results tend to contradict the influence of low N/P ratio.

Shapiro (1973) attributed the dominance of Cyanophyceae in fresh water reservoirs mainly to water

temperature, light, CO₂ and pH. Chellappa (1990)

ar-gued that the semi-arid reservoirs of northeast Brazil had suffered greatly from the effects of the environmental impact of cultural eutrophication, which favor the drastic increase in Cyanophyceae populations and cause reduced diversity in microalgae species. Branco and Senna (1996)

reported an abundant presence of Cylindrospermopsis

raciborskii in Paranoá Lake in Brasilia in the last

three decades of the 20th_{century, reaching densities of}

1.0x108 trichomes per liter that represented the main

cause of high phytoplankton densities. The presence and

the abundance of C. raciborskii were also reported for

a eutrophic ecosystem in São Paulo state for short and long-term periods (Tucci and Sant’ana, 2003). According

to Padisák (1997), Cylindrospermopsis raciborskii has

been considered an expanding species in various parts of the world, and presented potential toxicity and a tenden-cy to bloom formation. Its ecological success is due to various factors: migration capacity in the water column; tolerance to low luminosity; high capacity for phosphate and ammonium absorption; capacity to fix atmospheric nitrogen; herbivory resistance; and the ability of this alga to fix inorganic carbon at high pH values or the capac-ity to use bicarbonates directly as a source of carbon (Golterman et al., 1978).

The Cruzeta reservoir exhibits great phytoplank-ton diversity with a total of 90 taxa distributed into 6 classes. Chlorophyceae dominated most of the

time, the most frequent taxons were: Scenedesmus

quadricauda,Chlorellasp.,Eudorina sp., Pandorinasp.,

Dictyosphaerium sp. and Oocystis sp. Toxic

cyanobac-teria, such as Cylindrospermopsis raciborskii,

domi-nate occasionally and preferentially during the dry pe-riod, possibly as a result of increased temperature and

transparency. Bacillariophyceae, such as Aulacoseria

granulata, is frequent in the environment. The increase

in the Phacus acuminatus population (Euglenophyceae)

coincided with the increase in pH and the greater con-centrations of orthophosphate. The reservoir is charac-teristically an oligo-mesotrophic environment.

Acknowledgements — We thank CAPES/MEC for the scholarship awarded to one of us (J.L.M.B), CNPq (Proc. No.475619/2004-8) for the research Grant and UFRN for the overall facilities provided.

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